1. Field of the Invention
The present invention relates to the technical field of a rear-focus zoom lens device for carrying out zooming by a zooming lens, which is a combination lens, disposed at an end of a lens barrel, and for carrying out focusing by a focusing lens, which is a combination lens, disposed rearward of the zooming lens. The present invention also relates to the technical field of a video camera including the rear-focus zoom lens device.
2. Description of the Related Art
As is widely known, two types of zoom lens devices are used in, for example, video cameras. They are a front-focus zoom lens device and a rear-focus zoom lens device. In the front-focus zoom lens device, focusing is carried by a focusing lens, which is a combination lens, disposed at an end of a lens barrel, and zooming is carried out by a zooming lens, which is a combination lens, disposed rearward of the focusing lens. In the rear-focus zoom lens device, zooming is carried by a zooming lens, which is a combination lens, disposed at an end of a lens barrel, and focusing is carried out by a focusing lens, which is a combination lens, disposed rearward of the zooming lens.
In the front-focus zoom lens device which is the type of zoom lens device which is most often used in business video cameras, the position of a zoom ring, which is rotationally operated for zooming, in a rotational range is absolute, that is, is set within a predetermined angular range. Therefore, operation ends, that is, a zooming wide angle end and a zooming telephoto end, are definite, that is, the zoom ring is positionally restricted by a stopper. Consequently, the front-focus zoom lens device is excellent in, for example, adjusting the angle of view.
In recent years, even in business video cameras, there is a demand for easy operation and auto-focusing with progress in downsizing.
However, in a related front-focus zoom lens device used in a business video camera, focusing is carried out by a heavy, large diameter focusing lens disposed at an end of a lens barrel. Therefore, provision of an auto-focusing function for controlling the focusing lens by an electric operation results in problems, such as an increase in the size of an actuator and an increase in power consumption. Consequently, the front-focus zoom lens device is not necessarily a suitable zoom lens device.
In order to reduce costs, it is desirable to use a rear-focus zoom lens device which carries out focusing by a light, small-diameter focusing lens disposed at the rear end of a lens barrel.
In the rear-focus zoom lens device, however, when auto-focusing is utilized, the movements of the zooming lens and the focusing lens are controlled in the optical axis direction by an electronic cam in order to automatically carry out zooming and focusing, respectively, based on the relative positions of the lenses. Therefore, the positions of a zoom ring and a focus ring, which are rotationally operated, in rotational ranges are not absolute. Consequently, the positions are relative positions corrected by their positions relative to each other.
In other words, the zoom ring and the focus ring do not have absolute angular coordinates, as a result of which zooming and focusing are carried out based on relative coordinates based on how many angles the zoom ring and the focus ring are rotated and in which direction from their current positions. Therefore, the zoom ring and the focus ring rotate endlessly, and graduations for indicating the rotational positions of the zoom ring and the focus ring are not provided. Consequently, the operation ends of the zoom ring and the focus ring cannot be determined.
In a business video camera which requires a complicated image representation by, for example, repeating zooming within the same range by the rear-focus zoom lens device having this disadvantage, the fact that the zooming position is a relative position which changes each time focusing is carried out is a critical disadvantage. This is one of the major reasons why the rear-focus zoom lens device is not conventionally accepted for use in video cameras.
To overcome the aforementioned problems, a rear-focus zoom lens device which comprises a clutch mechanism for switching between manual function and auto function in order to determine the operation ends is provided (refer to, for example, Japanese Unexamined Patent Application Publication No. 2003-5015). Many patent applications regarding such a clutch mechanism have been filed. There is also a zoom lens device having a mechanical zoom ring. In this type of zoom lens device, when the zoom ring reaches either a focus or a zoom optical end, the rotation of the operated ring is stopped by rotationally driving a stopper by a motor and engaging it with one of the teeth of a gear of the zoom ring in order to allow an operator to confirm by touch that the zoom ring has reached either the focus or zoom optical end (refer to, for example, Japanese Unexamined Patent Application Publication No. 8-313793).
In the related clutch mechanism, however, when the zoom function is set to the manual mode once, if it becomes necessary to carry out flange back adjustment or angle-of-view correction by zooming due to thermal expansion/contraction of the entire lens barrel or due to an auto-focusing function, electrical correction cannot be carried out. In the rear-focus zoom lens device, since the control of the rotation of the zoom ring is electrically carried out, the operation of the zoom lens device is lightened compared to a front-focus zoom lens device having a mechanical zoom ring. Therefore, when an operator tries to carry out focusing, he/she may touch the zoom ring, or the zoom ring may move by, for example, slight vibration when it is installed in a vehicle. When focusing is carried out while the angle of view is deliberately fixed, such as in time-lapse shooting or animation shooting, the operator may accidentally touch the zoom ring and unconsciously change the angle of view, as a result of which an image cannot be used. Even if a related zoom lens device can store the position, it does not have a mechanism for fixing the position.